Some embodiments according to the invention are related to a digital-to-analog conversion arrangement. Some embodiments according to the invention are related to a method for digital-to-analog conversion of a digital input signal.
The field of wireless communications has seen a rapid evolution in the past, especially during the last two decades. During this evolution, a number of wireless communications standards were introduced, each new standard typically exploiting technological progress in order to offer superior performance. For example, the GSM (Global System for Mobile Communications), EDGE (Enhanced Data rates for GSM Evolution), and CDMA (Code Division Multiple Access) standards were and still are widely used by wireless communication networks and their subscribers. While early mobile stations such as cellular phones typically supported a single wireless communication standard only, subscribers were quick to demand more versatile mobile stations or devices which can be used with wireless networks operating under different wireless communications standards. Although the mobile device could comprise dedicated transceiver sections for the different communication standards, this typically leads to higher costs and increased space requirements. Therefore, multi-mode RF transceiver architectures are currently explored. Different standards like GSM/EDGE/UMTS define requirements to be fulfilled by a transceiver operating under the corresponding wireless communication standard.
Some transceiver architectures employ analog filters in a radio frequency (RF) domain in order to spectrally shape an outbound transmit signal or an inbound receive signal. Since the different wireless standards are assigned to different frequency ranges, a different filter would have to be used for each wireless standard. Since filters for the RF range are typically relatively expensive and bulky, transceiver designers attempt to find solutions that do not require an RF filter.
One component that can be found in transmitter architectures is a radio frequency digital-to-analog converter (RF-DAC), which is basically an up-converter combined with a digital-to-analog converter. When aiming at a multi-mode device for different standards like GSM/EDGE/UMTS, the RF-DAC needs to fulfill various requirements for error vector magnitude (EVM), TX-noise in the RX-band, and adjacent channel leakage rejection (ACLR). Some important RF-DAC requirements in an interstage filter-less polar transmitter are, for instance:
an output power gain control from −73 dBm up to 6 dBm (output power dynamic of 79 dB);
a local oscillator leakage requirement of −85 dBm for an output power of −67 dBm (see FIG. 1);
an output noise requirement of 160 dBc for output powers between −8 dBm up to 6 dBm, which is relaxed for lower output powers;
a third degree intermodulation (IM3) requirement of about −37 dB; and
change of power level in a transmission slot.
RF-DAC topologies that fulfill the specification for a multi-mode polar-transmitter application without interstage-filters are currently the subject of research in the mobile communications industry. In these filter-less RF-DAC topologies, the only available filtering between a low noise amplifier (LNA) of a transmitter receive path (RX) and an output port of the power amplifier in the transmit (TX) path is typically the duplexer attenuation. Especially filter-less DAC-mixer topologies, which fulfill an output noise requirement of 160 dBc as defined above for an offset-frequency of 20 MHZ up to 450 MHZ, have been and are still investigated. Other requirements to be fulfilled are the above mentioned LO-leakage and IM3 specification. At least some of these specifications may be partially contradicting each other and may lead to a conflict.